This invention relates to a fuel heating device and to a device which is suitable as the initial stage in a fuel evaporation system (the carburetor part) of an automobile using gasoline as its fuel. In the conventional fuel system of automobiles, gasoline is atomized, a suitable amount of air is supplied automatically if it is required, for converting the same into a combustible gas (mixed gas), and this combustible gas is then supplied to the cylinder of the engine. A carburetor is used for the purpose of obtaining such a mixed gas of air and gasoline. In some known systems, this mixed gas of atomized gasoline and air is preheated by a honeycomb heater as it is led to the cylinder of the engine. A large number of gas passageways are compactly formed in the honeycomb heater and, when the heater is given an electric current, the mixed gas that passes it is heated to a prescribed temperature so as to prevent knocking or to economize the fuel consumed. The honeycomb heater is ordinarily constructed so that the passage of electricity may stop when the temperature of the engine rises.
In some systems, a cylindrical down tube heater is also provided below the honeycomb heater to be used for the evaporation of the liquid fuel that flows down along the surface of the inner wall of the mixed gas passageway. When this down tube heater is activated, the honeycomb heater is not used, and when the honeycomb heater is operated, on the other hand, the down tube heater will not be activated, only one of the heaters being operated in ordinary cases.
The down tube heater that is used here is typically such that a dual tube consisting of an inner tube and an outer tube made of a metal is formed, with a heater (such as a PTC ceramic heater) being charged between these tubes. In such a heater, where the outer tube is formed by a metal whose heat conductivity is high, the heat coming from the heater that is charged between the two tubes tends to be dispersed unnecessarily outside of the heater (outer peripheral direction), with a result that the thermal efficiency for fuel heating on the side of the inner periphery which is important becomes unsatisfactory. The heating efficiency on the side of the inner periphery is not satisfactory and the efficiency of such heaters is low.